A Waking Nightmare

1st You should go to the Windpeak Inn, which is right next to the Dawnstar Barracks on the "up" part of the city (Dawnstar).

2nd Talk to Erandur, he'll ask you to follow him to the Nightcaller Temple, where you'll have to talk to him again in order to really begin because only he can open the place where you actually need to go.

3rd You'll follow Erandur to a few places and kill a few of his friends that were asleep, untill you arrive at a kind of spell made shield that will be blocking your way, then you will follow to the library. Oh yeah... careful not to kill him.

4th You should talk to him, he'll ask you to find a book called "The Dreamstride", you, then, will have to tell him you found it and follow to the laboratory, where you'll have to find the Vearmina's Torpor and talk to Erandur again.

5th Then you'll have to drink the potion, that will make you go into some kind of dream, wihch you'll be seeing things through Erandur's eyes, where you'll have to release a gas named miasma and disable the magical barrier.

6th As soon as you wake up, you'll see Erandur, and you should talk to him, follow him to the skull of corruption and defeat Veren and Thorek.

7th Again talk to Erandur and wait for him to dispel the barrier, then you'll be given two options, either you kill him and take the skull of corruption or you wait for him to finish the ritual, talk to him and gain a new partner on your journey.

Hum... if you're into those things you may have both, letting Erandur finish the ritual and clicking the key under esc and on the right side of 1, and typing "player.additem 00035066" <# of skulls you wish to have>.